THIS INVENTION relates to a method of synthesizing an essentially single phase lithium manganese oxide in accordance with the formula Li.sub.1-x Mn.sub.2 O.sub.4 in which O.ltoreq.x&lt;1 and having a spinel-type crystal structure. In particular, the invention relates to a method of synthesizing such oxide to produce an oxide which is suitable for use as a cathode in an electrochemical cell of the Li/Li.sub.y MnO.sub.2 type, with an anode comprising lithium or a suitable lithium-containing alloy. The invention also relates to the oxide when produced by the method; and to an electrochemical cell comprising said oxide as its cathode.
According to the invention, a method of synthesizing a lithium manganese oxide having a spinel-type crystal structure comprises forming a mixture in finely divided solid form of at least one lithium salt is defined herein and at least one manganese salt as defined herein, and heating the mixture in an oxidizing atmosphere to a temperature in the range 200.degree.-600.degree. C. to cause said salts to react with each other and to obtain said lithium manganese oxide having a spinel-type crystal structure by simultaneous decomposition and cubic close packed oxygen lattice construction.
Certain forms of the lithium manganese oxide having a spinel-type structure produced by the method can be expressed by the formula Li.sub.1-x Mn.sub.2 O.sub.4 in which O.ltoreq.x&lt;1, but it is to be noted that this Li.sub.1-x Mn.sub.2 O.sub.4 can have a (Mn.sub.2)O.sub.4.sup.n- framework structure in which the quantity of Mn cations varies from the stoichiometric value.
By a `lithium salt as defined herein` is meant a lithium salt which decomposes when heated in air to form an oxide of lithium an, correspondingly, by a `manganese salt as defined herein` is meant a manganese salt which decomposes when heated in air to form an oxide of manganese.
The salt of lithium may be a member of the group consisting of Li.sub.2 CO.sub.3, LiNO.sub.3 and mixtures thereof, the salt of manganese being a member of the group consisting of Mn(NO.sub.3).sub.2, MnCO.sub.3 and mixtures thereof. Forming the mixture may be in a stoichiometric ratio so that there is an at least approximate molar ratio of Li:Mn of 1:2, optionally with a slight excess of either salt, i.e. such that the ratio is 1:1.7-1:2.5, preferably 1:1.9-1:2.1. Forming the mixture may be by milling, e.g. in a ball mill containing alumina grinding media or in a mortar and pestle so that the mixture has an average particle size of at most 250 microns. Instead, forming the mixture of the lithium and manganese salts may be by making a slurry in a solvent selected from the group consisting of water, ethanol and mixtures thereof and thereafter drying the mixture until the solvent content is at most 10% by mass, e.g. by drying at 60.degree.-90.degree. C. in a drying oven for 12 hours.
The heating of the mixture may be in air to a temperature of 300.degree.-420.degree. C., e.g. 400.degree. C.; the mixture being held at the maximum temperature, preferably with an accuracy of .+-.10.degree. C., for a period of at least 2 hrs, e.g. 2-5 hours. Heating may typically be at a rate of 60.degree. C./hr; and may be followed by cooling by quenching in air or slow cooling at the natural furnace cooling rate. The heating may be of the mixture in powder form. However, the method conveniently includes the step of, prior to the heating, compacting the mixture, by pressing it at a pressure of 5-10 MPa to form a unitary artifact, so that, after the heating, the lithium manganese oxide of the formula Li.sub.1-x Mn.sub.2 O.sub.4 is in the form of a self-supporting unitary artifact.
Spinel compounds have structures that can be represented by the general formula A(B.sub.2)X.sub.4 in which X atoms are arranged in a cubic-close -packed fashion to form a ne.g.atively charged anion array comprised of face-sharing and edge-sharing X tetrahedra and octahedra. In the formula A(B.sub.2)X.sub.4, the A atoms are tetrahedral-site cations and the B atoms are octahedral-site cations, i.e. the A cations and B cations occupy tetrehedral and octahedral sites, respectively. In the ideal spinel structure, with the origin of the unit cell at the centre (3m), the close-packed anions are located at the 32e positions of the space group Fd3m. Each unit cell contains 64 tetrahedral inerstices situated at three crystallographically non-equivalent positions 8a, 8b and 48f, and 32 octahedral interstices situated at the crystallographically non-equivalent positions 16c and 16d. In an A(B.sub.2)X.sub.4 spinel the A cations reside in the 8a tetrahedral interstices and the B cations in the 16d octahedral interstices. There are thus 56 empty tetrahedral and 16 empty octahedral sites per cubic unit cell. For the present invention A(B.sub.2)X.sub.4 is represented by Li(Mn.sub.2)O.sub.4.
Therefore, the B cations of the (B.sub.2)X.sub.4.sup.n- framework structure may be re.g.arded as being located at the 16d octahedral positions and the X anions located at the 32e positions of the spinel structure. The tetrahedra defined by the 8a, 8b and 48f positions and octahedra defined by the 16c positions of the spinel structure, thus, form the interstitial space of the (B.sub.2)X.sub.4.sup.n- framework structure for the mobile Li cations, for diffusion therethrough during the electrochemical discharge and charge reactions.
Furthermore, the cathodes of the present invention need not necessarily be stoichiometric compounds. For example, cathodes may be synthesized in which defects are created by varying the quantity of Li ions at the A sites to generate a lithium-defici.e.nt spinel Li.sub.1-x (Mn.sub.2)O.sub.4 with 0.ltoreq.x&lt;1; alternatively cathodes may be synthesized in which defects are created by varying the quantity of Mn cations in the framework structure such that additional Li cations may enter the framework. In certain instances, these additional Li cations may partially occupy the 16d octahedral sites normally occupi.e.d by the Mn cations. Under such circumstances, these partially occupi.e.d octahedra may be considered to form part of the interstitital space. Conversely, cathodes may also be synthesized, in which part of the interstitial spaces defined by the 8a, 8b and 48f tetrahedral and 16c octahedral interstices of the spinel structure may be occupi.e.d by Mn cations, thereby rendering these particular sites at least partially inaccessible to the mobile Li cations. It follows that, in compounds of the formula Li.sub.1-x Mn.sub.2 O.sub.4 of the lithium manganese oxide synthesized by the method of the present invention, the Mn:O atomic ratio need not be precisely 1:2, but will be about 1:2, so that the formula Li.sub.1-x Mn.sub.2 O.sub.4 is defined as covering also compounds in which the Mn:O ratio is slightly greater than 1:2 and compounds in which said ratio is slightly less than 1:2, the formula Li.sub.1-x Mn.sub.2 O.sub.4 merely being used for convenience of expression.
The Li.sub.1-x Mn.sub.2 O.sub.4 spinel-type oxide product of the present invention can be described, broadly, as Li.sub.y MnO.sub.2 in which y is not greater than 0.5. When this Li.sub.y MnO.sub.2 is assembled into a cell of the type Li (anode)/electrolyte/Li.sub.y MnO.sub.2 (cathode), charging will involve a reduction of the value of y to a theoretical minimum value of 0 in the fully charged state. While the electrolyte may be a lithium-containing molten salt electrolyte, it is conveni.e.ntly a room-temperature electrolyte such as LiClO.sub.4, LiAsF.sub.6 or LiBF.sub.4, dissolved in an organic solvent such as propylene carbonate or dimethoxyethane. It is also in principle possible to discharge such cells further than a partially discharged state in which y in LiyMnO.sub.2 is 0.5, up to a practically useful value for y of 1. Although y values in excess of 1 are possible, the electrochemical reaction when y exceeds 1.0 will be associated with a sharp drop in voltage from an open circuit value of approximately 3 V to below 2 V, which limits usefulness. In practice, the value of y will be controlled between conveni.e.nt values, e.g. a value of y=0.2 in the nominally fully charged state and a value of y=1.0 in the nominally fully discharged state.
The invention also extends to lithium manganese oxide in ( accordance with the formula Li.sub.1-x Mn.sub.2 O.sub.4 whenever synthesised by the method described above, particularly for use as a cathode in an electrochemical cell.
The invention also extends to an electrochemical cell having a cathode comprising a lithium manganese oxide of formula Li.sub.1-x Mn.sub.2 O.sub.4 as described above, an anode which comprises lithium metal or a lithium-containing alloy, and an electrolyte whereby the anode is electrochemically coupled to the anode.
The cell may, thus, be of the type described above.
The cells of the invention may be primary cells or rechargeable secondary cells. Secondary cells can in principle be loaded with the Li.sub.1-x Mn.sub.2 O.sub.4, i.e. Li.sub.y MnO.sub.2 in which y is not more than 0.5, but may be loaded, if more conveni.e.nt, with y at some other value. In practice the Li.sub.1-x Mn.sub.2 O.sub.4 of the cathode will usually be compacted, as mentioned above, optionally with a suitable binder, and for cathode use a suitable electronically conductive material to act as current collector may be incorporated in the compacted cathode. Thus, polytetrafluoroethylene (PTFE) may be used as a binder and acetylene black (carbon) may be used as current collector.
Accordingly, in a particular embodiment of the cell, the lithium manganese oxide of the cathode may be present in the cathode in admixture with a binder and with an electronically conductive current collector in a compacted artifact, the electrolyte being a room-temperature electrolyte comprising a member of the group consisting of LiClO.sub.4, LiAsF.sub.6 and LiBF.sub.4 dissolved in an organic solvent selected from the group consisting of propylene carbonate and dimethoxyethane.
Without being bound by theory, the Applicant believes that an advantage of the present invention, whereby particular utility of the Li.sub.1-x Mn.sub.2 O.sub.4 as a cathode of a cell of the type in question is obtainable, arises from the relatively poorly developed crystallinity and high surface area thereof as discussed in more detail hereunder. This poorly developed crystallinity is to be contrasted with the well developed crystallinity obtained when a lithium salt such as Li.sub.2 CO.sub.3 is heated in a similar fashion in air with, for example, MnO.sub.2, Mn.sub.2 O.sub.3 or Mn.sub.3 O.sub.4, to a temperature in the range of 700.degree. C. to 900.degree. C. The advantage of using e.g. Mn(NO.sub.3).sub.2 or MnCO.sub.3 precursors in the reaction as opposed to manganese oxides, such as those mentioned above, is that the nitrate or carbonate compounds decompose rapidly within a period of 2-5 hours at relatively lower temperatures, particularly in the presence of the lithium salt, to produce a virtually amorphous intermediate, which can be expressed as Li.sub.2 0.4 MnO from which the Li.sub.1-x Mn.sub.2 O.sub.4 product, and in particular, its cubic close packed oxygen lattice, must be reconstructed. This accounts for the poorly developed crystallinity and strain in the individual particles. When starting from manganese oxide precursors e.g. as mentioned above, it is believed that the retention of much of the oxygen content makes it more difficult to obtain a single-phase Li.sub.1-x Mn.sub.2 O.sub.4 product at these relatively low temperatures, with the required electrochemical properties.
The oxidizing atmosphere under which the heating takes place may be an oxygen-containing atmosphere, conveniently air.